The invention relates to a method of recording a high-definition television (HDTV) signal on a storage medium which is arranged for storage of a standard-definition television (SDTV) signal, the method comprising the steps of generating a sub-sampled version of the HDTV signal, recording said sub-sampled version of the HDTV signal in first locations of the storage medium which represent an SDTV image, compressing the HDTV signal to obtain a compressed HDTV signal, and recording the compressed HDTV signal in further locations of the storage medium.
The invention also relates to a method of transmitting an HDTV signal and to an apparatus for formatting an HDTV signal so as to allow recording thereof on SDTV recorders, and to a method and arrangement for playing back HDTV signals.
A known method of recording HDTV signals on a conventional SDTV recorder is disclosed in Applicant""s United States Patent U.S. Pat. No. 5,646,692. In this prior-art method, the HDTV signal is compressed in a manner which is well-known in the art. More particularly, each image is divided into blocks of 8*8 pixels, and the blocks are subjected to a Discrete Cosine Transform to obtain a DC coefficient and a plurality of AC coefficients for each block. The AC coefficients are quantized and variable-length coded, the quantization being controlled in such a way that the amount of data for each image corresponds to the amount of data of an uncompressed SDTV image. The data is then recorded on the SDTV recorder.
In one embodiment of the prior-art method, the SDTV recorder is of a type which stores uncompressed SDTV signals. In this embodiment, the DC coefficients of the HDTV signal (which represent the average luminance or chrominance of the blocks) are recorded as SDTV pixels. The DC coefficients are ordered in such a way that the corresponding SDTV pixels constitute a small but recognizable version of the HDTV picture. The rest of the compressed HDTV signal is recorded outside this SDTV sub-picture and becomes manifest as noise or snow when the recorded data is reproduced by an SDTV receiver. The recorder used in this embodiment is a professional machine and is relatively expensive.
In another embodiment of the prior-art method, which corresponds to the preamble of the appended claims, the SDTV recorder is of a type which stores compressed SDTV signals. In this embodiment, the DC coefficients of the HDTV signal are first arranged to constitute the SDTV sub-picture. This sub-picture is then applied to the recorder and stored, using the recorder""s compression stage. The compressed HDTV signal bypasses the recorder""s compression stage and is directly recorded on tape locations not used for storing the sub-picture.
The prior-art method allows HDTV material to be recorded on, and edited with, SDTV equipment.
It is an object of the invention to further improve the above acknowledged prior-art method for recording HDTV signals.
To this end, the method in accordance with the invention is characterized in that the storage medium is of a type which comprises storage blocks for storing compressed SDTV image blocks, the method further comprising the steps of recording the sub-sampled version of the HDTV signal in first locations of selected storage blocks, recording an end-of-block code after said first locations, and recording the compressed HDTV signal in further locations of said storage blocks.
The invention is based on the desire to use cost-effective and readily available SDTV recorders for recording and editing MPEG2-coded HDTV material. Examples of such SDTV recorders are known in the art as DVCPRO or digital Betacam. They store compressed image SDTV blocks in respective storage blocks. With the invention, it is achieved that the sub-sampled version of the HDTV signal is recorded in only a part of said storage blocks. The end-of-block codes ensure that the data stored in further locations of the storage blocks is ignored when the recorded signal is reproduced by the SDTV recorder. The capacity for recording the HDTV signal is thus enlarged.
In one embodiment of the method, selected DC coefficients of the compressed HDTV signal are recorded in storage block locations which represent the DC coefficients of the SDTV signal. When the recorded signal is reproduced by SDTV equipment, the selected DC coefficients are displayed as image blocks having a uniform luminance and chrominance. They constitute a full-screen SDTV image, showing a checked but recognizable version of the HDTV image, which is sufficient for the purpose of editing. In this embodiment, the DC coefficients of all storage blocks collectively constitute the sub-sampled version of the HDTV signal.
In another embodiment the HDTV signal is first sub-sampled in the spatial domain to form an SDTV sub-picture. Said sub-picture is then separately encoded to obtain compressed image blocks complying with the format of, but having fewer bits than, the storage blocks of the storage medium. In this embodiment, the sub-sampled HDTV signal is reproduced as an SDTV sub-picture at normal resolution.